Al Gratín Recipe

Prep and par-cook

Preheat oven to 375°F (190°C). Peel the potatoes (optional) and slice them into 1/8"–3/16" (3–5 mm) rounds using a sharp knife or mandoline so they cook evenly. Place slices in a large bowl of cold water to remove excess starch for 5 minutes, then drain and pat dry with a clean towel.

Bring a large pot of water to a gentle boil, add 1 tablespoon kosher salt, then add the potato slices. Simmer gently for 5 minutes — they should be just beginning to become tender but still hold shape (par-cooked). Drain in a colander and let steam-dry for 2–3 minutes.

Butter a 9x9-inch (or similar 8x10-inch) baking dish with 1 tablespoon unsalted butter, coating bottom and sides.

Make roux and infuse

In a medium saucepan over medium heat, melt 4 tablespoons unsalted butter. Add 4 tablespoons flour and stir constantly with a whisk for 1.5–2 minutes until the mixture turns a pale golden and loses the raw flour smell (do not brown).

Slowly pour in about 1/2 cup of the cold milk while whisking to form a smooth paste, then continue adding the remaining milk in a steady stream, whisking to keep the sauce smooth and lump-free.

Add the crushed garlic clove to the sauce and bring to a gentle simmer over medium-low heat, whisking occasionally. Cook until the béchamel thickens to coat the back of a spoon, about 6–8 minutes. Remove and discard the garlic clove.

Season the béchamel with 1 teaspoon salt, 1 teaspoon freshly ground black pepper, and 1 teaspoon freshly grated nutmeg. Taste and adjust seasoning — remember cheeses add salt, so be conservative.

Layering

Place a single even layer of potato slices covering the bottom of the buttered dish (overlap slightly). Spoon a thin layer (~1/3 cup) of béchamel over the potatoes and sprinkle a light handful (about 1/4 cup) of the Gruyère and a couple tablespoons of white cheddar and a tablespoon of Parmesan. Repeat layering — potatoes, béchamel, cheeses — making 3–4 layers until potatoes are used, finishing with béchamel on top.

Reserve about 1/4 cup Gruyère, 2 tablespoons white cheddar and 2 tablespoons Parmesan to sprinkle on the top layer. If using panko, mix the 1/2 cup panko with the reserved cheeses and scatter evenly over the final béchamel layer for a crisp topping.

Baking

Cover the dish loosely with foil (shiny side up) and bake in the preheated 375°F (190°C) oven for 30 minutes. Remove the foil and continue to bake for an additional 20–25 minutes until the top is golden brown, the cheeses are bubbling, and a thin knife inserted into the center meets little resistance.

If the top hasn't browned to your liking, broil on high for 1–2 minutes, watching carefully to avoid burning.

Remove from oven and let rest 10–15 minutes to set. Sprinkle with fresh thyme or chives and a light dusting of cracked black pepper before serving.